Phenol Red Broth Base and Phenol Red Broth with carbohydrates are used for the determination of fermentation reactions in the differentiation of microorganisms.
SUMMARY AND EXPLANATION
The ability of an organism to ferment a specific carbohydrate incorporated in a basal medium, resulting in the production of acid or acid and gas, has been used to characterize a specific species or group of bacteria, aid in the differentiation between genera and aid in species differentiation.
In 1950, Vera recommended using pancreatic digest of casein in fermentation test media. She found that casein peptone could be used with the pH indicator phenol red in fermentation tests with a high degree of accuracy.
Phenol Red Broth Base is a complete medium without added carbohydrate. It is used as a negative control for fermentation studies or as a base for the addition of carbohydrates. Pancreatic digest of casein provides nutrients and is low in fermentable carbohydrate. The pH indicator, phenol red, is used to detect acid production.
Phenol Red Broths, prepared with a final concentration of one-half percent carbohydrate, are convenient for the determination of fermentation reactions. Most of the end products of carbohydrate fermentation are organic acids which, in the presence of phenol red, produce a color change in the medium from red to yellow. If gas is produced during the fermentation reaction, it is collected in the inverted Durham tube.
No yellow color should occur in the control tube. If it does, the results cannot be correctly interpreted since acid has been produced without fermentation.